The present invention relates to a device for changing propeller pitch by using an electrohydraulic actuator. It is applicable to any system provided with a propeller, e.g. a wind generator or an industrial fan, as well as to turboprops and to turbojets provided with a fan comprising a plurality of blades that can be set at variable angles.
In order to increase the performance and improve the efficiency of a turbomachine engine throughout all of its operating stages, from takeoff to landing, it is known to have recourse to varying the pitch of a propeller or a fan. Such variable settings also make it possible to vary the speed of the propeller or the fan to obtain the desired thrust without varying the speed of the turbine which is in general set to its maximum continuous speed, or indeed during landing to reverse said thrust, thus replacing conventional thrust reversal systems that are heavy and complex.
Present devices for varying propeller pitch generally comprise hydraulic actuators (one actuator driving each blade root) which deliver the force needed to place the propeller in the desired position. Nevertheless, those devices are particularly complex and bulky, and they also present major problems of reliability, due in particular to using rotary couplings or joints. It is important to avoid any hydraulic fluid leakage into the turbine since that will degrade its performance, or can even set it on fire, and it is important to limit any risk of pollution (contaminating air intakes). U.S. Pat. No. 5,897,293 is a good illustration of that type of hydraulic device comprising very numerous pipes, hydraulic valves, and rotary joints which need to pass through or along the body and the drive shaft of the turbomachine.
Propeller pitch can also be controlled by an electrical drive system based on permanent magnet motors or an asynchronous motors associated with screw-and-nut systems (see for example EP 0 154 808) or with respective gearwheels secured to each of the blades (FR 890 641 and FR 2 712 250, for example). Nevertheless, such systems need to resort to motors that are bulky in order to deliver the high levels of power needed to change propeller pitch quickly without excessively gearing down the pitch-setting mechanism.
The present invention seeks to mitigate the drawbacks of those prior art devices with a propeller pitch control device that is simple, compact, and particularly reliable. An object of the invention is specifically to propose a control device which eliminates any risk of hydraulic leakage.
These objects are achieved by an electrohydraulic device for varying the pitch of the blades of a machine rotor, the blades being mounted on a hub rotated by a drive shaft of the machine, and each blade having a root capable of pivoting about a longitudinal axis of the blade, the drive shaft itself being rotated relative to a stationary structure of the machine, the device comprising firstly a hydraulic actuator whose piston is secured to a control and synchronization ring in which off-center wrist pins are engaged, each fixed in register with one of said blade roots, and having a cylinder closed by a cover that includes a high pressure hydraulic pump (preferably a piston pump) feeding hydraulic fluid to both faces of said piston via channels integrated in said cylinder of the actuator, and secondly an electric motor controlled and powered by an electricity generator comprising an inductor secured to said stationary structure of the machine and an armature secured to said drive shaft of the machine.
This structure leads to a highly simplified control device that is particularly compact and easy to implement. Any risk of leakage is greatly reduced because of the absence of any rotary couplings and pipework.
The electricity generator may be constituted by a rotary transformer or by a collector system comprising smooth slip rings and brushes. It is actuated by an electronic control circuit connected firstly to a machine power supply device and secondly to a machine computer, and arranged to modulate the electricity supplied to said electric motor as delivered by said power supply device so as to act on the operating speed and/or time, and the direction of rotation of said high pressure hydraulic pump in order to obtain determined displacement of the piston corresponding to the setting desired for the rotor blades.
The hydraulic actuator preferably includes a linear variable displacement transducer (LVDT) type sensor constituted by a cylindrical tube secured to the cylinder of the actuator and a plunging core secured to a piston rod and capable of moving linearly in the tube.
The invention is equally applicable to the fan of a turbojet and to the propeller of a turboprop, to a wind generator, or to an industrial fan. Advantageously, in the field of aviation, the computer is the full authority digital engine computer (FADEC) of the turboprop or the turbojet.